Generally in the process for making paper, incremental amounts of liquid are removed from a slurry of pulp in a succession of steps. In a first forming step, the slurry is deposited on a porous forming fabric which drains much of the liquid by gravity and suction, and leaves a wet web of solids on the fabric surface. In a later pressing step, the wet web is compressed while on a press fabric in order to removed additional liquid. In a still later, drying step, more liquid is removed by evaporation, usually by supporting the web on a dryer fabric so that the web is in contact with large diameter, smooth, heated rolls.
The papermaking process places considerable demands on the fabrics used in each process step. The fabric should be structurally strong, flexible, abrasion resistant, chemical resistant, and able to withstand any temperatures for extended times.
A major improvement in the technology of papermaking fabric has been the introduction of synthetic polymer monofilaments. A suitable polymer must provide a yarn having chemical and physical properties which satisfy the requirements of automated fabric manufacturing and the demands of papermaking.
Monofilaments have been made from such polymers as polyethylene terephthalate (PET). PET has good dimensional stability, reasonable resistance to abrasion and is moderately priced. However, it has marginable hydrolytic stability and it degrades rapidly in the presence of caustic solution.
The cost of a yarn can be decreased by including a certain amount of recycled PET. However the addition of such recycled material decreases the hydrolytic stability of the yarn and further decreases its dry heat stability.